1. Field of the Invention
The present invention relates to a lead-free and lithium-free optical phosphate glass, to the use of a glass of this type in the fields of imaging, projection, telecommunications, optical communication technology and laser technology, and also to optical elements or preforms for such optical elements made from this type of glass.
2. Description of the Related Art
In recent years, the market trend in both optical and opto-electronic technologies (the application areas of imaging, projection, telecommunications, optical communication technology and laser technology) has been ever more towards miniaturization. This is evident from the ever-decreasing size of end products and of course requires increasing miniaturization of the individual parts and components of end products of this type. For producers of optical glasses, this development, despite increasing numbers of end products, is associated with a considerable drop in the volume demand for raw glass. At the same time, there is an increasing pressure on prices from the further processors on the glass manufacturers, since the production of smaller components of this type from glass in ingot or bar form entails a significantly greater scrap percentage, based on the product, and the processing of extremely small parts of this type requires a higher outlay than larger components.
Instead of separating glass portions for optical components out of glass in ingot or bar form, as has hitherto been customary, in recent times production processes in which near net shape preforms, such as for example gobs, can be obtained directly from the glass melt. By way of example, there is an increase in demand from the further processors for near net shape preforms for re-pressing, known as precision gobs. Precision gobs are generally understood to be preferably completely fire-polished, semi-free-formed or free-formed glass portions, which have already been divided into portions and have geometry close to the final shape of the optical component.
Precision gobs of this type can advantageously also be transformed into optical elements such as lenses, aspherical components, etc. by the process known as precise pressing or precise molding. There is then no longer any need for further processing of the geometric shape or the surface, for example by surface polishing. This process can flexibly adapt to the smaller volumes of glass melt (distributed between a large number of small pieces of material) by virtue of having short set-up times. On account of the relatively small number of cycles or pieces and given the generally small geometries, the added value of the process, however, cannot originate solely from the value of the materials. Consequently, the products have to leave the press in a state, which is “ready for system installation”, i.e. it must be possible to dispense with complex reworking, cooling and/or cold re-machining. On account of the high geometric accuracies required, precision equipment with high-quality and therefore expensive mold materials have to be used for a pressing process of this type. The service lives of molds of this type form a huge part of the economics of the products and/or materials produced. An extremely important factor for achieving a high service life of the molds is for the operating temperature to be as low as possible, but this temperature can only be reduced to an extent, which still leaves the viscosity of the materials to be pressed sufficient for the pressing operation to be carried out. Therefore, there is a direct causal link between the working point and therefore the transformation temperature Tg of a glass, which is to be processed and the economics of a pressing operation of this type: the lower the transformation temperature of the glass, the higher the service lives of the molds and the greater the profit margin. This relationship results in the need for what are known as “low-Tg glasses”, i.e. glasses with low melting and transformation points, that is to say glasses, which have a viscosity that is sufficient for processing at the lowest possible temperatures.
A further objective, which has been reported with regard to the process engineering of the melt, has been a recent increased demand for “short” glasses, i.e. for glasses whose viscosity varies considerably with a relatively minor change in the temperature within a certain viscosity range. In the melting process, this behavior has the advantage that the hot-forming times, i.e. the mold closure times, can be reduced. This, on the one hand, increases the throughput, i.e. reduces the cycle time, and, on the other hand, is also gentler on the mold material, which, as has been described above, likewise has a positive effect on the overall production costs. “Short” glasses of this type have the further advantage that on account of the more rapid cooling compared to correspondingly longer glasses it is even possible to process glasses with a relatively high tendency towards crystallization. This avoids preliminary nucleation, which could cause problems in subsequent secondary hot-shaping steps, opening up the possibility of also allowing glasses of this type to be drawn to form fibers.
Furthermore, it is also desirable for the glasses, in addition to the optical properties mentioned and required, to be producible from components that are as inexpensive as possible and to be sufficiently chemically resistant.
Although the prior art has already described glasses with a similar optical position or comparable chemical composition, these glasses have considerable drawbacks. In particular, many of the glasses contain relatively high levels of the relatively expensive component Li2O and/or of the components, which increase the tendency towards crystallization, such as TiO2.
EP 1 275 622 relates to a glass for pressed bodies with a low softening point. The latter property is achieved by adding very high levels of alkali metals and relatively little P2O5. The glass must contain at least 6 percent by weight of Li2O.
JP 09-301735 describes an optical glass likewise with a low softening point. In this case too, this property is achieved by adding very high levels of alkali metals and relatively little P2O5. The glass has to contain a considerable amount of both Li2O and TiO2.
JP 2002-173336 describes an optical glass with a high refractive index for precise pressing technology. The glass must contain at least 2 percent by weight of Li2O and must also contain the components WO3, Nb2O5 and/or TiO2, which are likewise expensive. U.S. Pat. No. 5,053,360 and U.S. Pat. No. 4,875,920 describe ion-exchangeable glasses which always contain at least 5 percent by weight of Li2O.
JP 61-036137 describes a glass with a low melting point; the examples only mention glasses, which contain at least 4 percent by weight of Li2O.
JP 09-278479 describes a low-melting glass, which contains at least one percent by weight of Li2O. Moreover, Y2O3, La2O3 and/or Gd2O3 are mandatory constituents. These are likewise expensive components.
U.S. Pat. No. 6,409,396 describes a glass substrate which, when coated with interference layers, produces an interference filter. The only example cites a glass, which is not lead-free and furthermore also does not contain either BaO or ZnO.
JP 11-349347 describes a crystalline glass composition with a low melting point. It contains at least 0.1 mol percent of SnO2 which functions, inter alia, as an opacifier in the glass. SnO2 only melts at very high temperatures in relatively large quantities in oxide glasses and therefore makes the melting process more difficult.
WO 94/08373 (corresponding to U.S. Pat. No. 5,526,369), JP 63-021240 and DE 33 40 968 relate to laser glass, which must contain laser-active components, such as lanthanoides, and are therefore unsuitable for use as optical glasses.
DE 27 53 219 describes a fogging-free glass, which always contains from 6 to 15 mol percent of SiO2 and/or B2O3. Both components increase the liquidus point (upper devitrification point, UDP, German: “obere Entglasungsgrenze”), i.e. the temperature range above which no crystals can form in the glass or crystals which are present are dissolved again.
DE 1 596 854 relates to an optical glass with a substantially temperature independent optical path length. The examples cite exclusively glasses, which contain a sum of M2O of less than 15 percent by weight.
DE 1 089 934 describes an optical crown glass with a low dispersion. A refractive index position of from 1.50 to 1.57 at a dispersion of between 61 and 70 is not reached, however.
U.S. Pat. No. 2,381,925 describes a technical-grade glass with a high chemical resistance which has a P2O5 content of at least 60 percent by weight. Such a high level of P2O5 can no longer be added to the batch as a complex phosphate, but rather has to be added as free P2O5, which results in drawbacks in the melting properties during evaporation and dusting and produces a glass with a poor internal quality.
The same applies to the glasses described in JP 03-218941 for a polarizer in high-energy laser systems containing at least 60 percent by weight of P2O5. The optical glass described in U.S. Pat. No. 5,824,615 for precise-pressing technology also contains at least 73.9 percent by weight of P2O5 and also at least 1.1 percent by weight of Li2O.
DE 1 496 064 describes an optical glass, which contains at least 1 percent by weight of B2O3 and/or TiO2 with the drawbacks outlined above.
DD 29 825 describes a glass with a low refractive index and a high dispersion. It has to contain relatively large quantities of F and TiO2, inter alia, in order to set the optical position, in particular an Abbé number of less than 40. F is a component, which can make the production process more difficult on account of the ease of evaporation.
EP 0 481 166 relates to an optical glass which contains a relatively high ZnO fraction of 34 mol percent. Such a high level of ZnO increases the tendency of the glass to become opaque.
JP 02-124743 relates to an optical glass for precise pressing technology in which, however, the sum of components MO is well below 28 percent by weight.
JP 08-183632 describes a low-melting glass with phosphate content of at most 35 mol percent. It also contains at least 8 mol percent of B2O3. During melting, problems may arise in so far as a considerable proportion of B2O3 may be volatilized. Moreover, B2O3 makes the glass “long” in terms of its viscosity properties and also increases the UDP.